Electrical device



Nov. 4, 1969 c. J. B. FINCHAM 3,476,557

ELECTRICAL DEVICE Filed Dec. 31, 1964 FIGURE l FIGURE 3 United StatesPatent 3,476,557 ELECTRICAL DEVICE Christopher J. B. Fincham, Boston,Mass, assignor to National Research Corporation, Cambridge, Mass., acorporation of Massachusetts Continuation-impart of applications Ser.No. 3,896, Jan. 21, 1960, and Ser. No. 38,818, June 27, 1960. Thisapplication Dec. 31, 1964, Ser. No. 425,376

Int. Cl. B2213 3/10; H01g 9/05 US. Cl. 75-222 3 Claims ABSTRACT OF THEDISCLOSURE This invention relates to an electrical device and moreparticularly to a porous capacitor anode and the method of itsmanufacture. This application is in part a continuation of my copendingapplications Ser. No. 3,896, filed Jan. 21, 1960 and Ser. No. 38,8l8,filed June 27, 1960, both now abandoned.

It is a principal object of the present invention to provide a method ofmanufacturing an improved capacitor anode.

Another object of the present invention is to provide a. method ofproducing a sintered tantalum pellet having a low density and extremelyhigh porosity for use as an anode in electrolytic capacitors.

Another object of the present invention is to provide a method ofproducing a tantalum anode having an extremely high capacitance per unitvolume and per unit weight.

A still further object of the present invention is to provide a simpleprocess for forming a tantalum anode from tantalum powder of extremelysmall particle size for use in the production of a capacitor having ahigh capacitance per unit volume of tantalum together with a lowequivalent series resistance (hereinafter referred to as ESR).

A still further object of the invention is to provide a process forproducing tantalum anodes having very high capacitance with goodreliabiliy and with the other electrical characteristics beingsatisfactory.

Still another object of the invention is to provide capacitor anodeshaving extremely high reliability with reasonably high capacitance.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the product possessing the features,properties, and the relation of components and the process involving theseveral steps and the relation and order of one or more of such stepswith respect to each of the others which are exemplified in thefollowing detailed disclosure and the scope of the application of whichwill be indicated in the claims.

In practicing the present invention, in a preferred form thereof, thecapacitor anode is formed by introducing a finely-powdered, film-formingmaterial (such as tantalum or niobium) into a mold and heating thepowder to form a coherent self-supporting pellet. The first heating stepis preferably carried out under conditions which do not completely formthe anode but only provide sufiicient cohesion between the particles sothat the pellet can be removed and handled as a unit. Thereafter thepellet can be removed from the mold and subjected to a further sinteringtreatment under vacum during which the pellet is heated to a temperatureof 1875 C. and above to form a sintered, porous pellet which can beanodized to provide a highcapacitance, low-leakage anode. In a preferredembodiment of the invention, the initial sintering takes place for about20 minutes at a temperature of about 1400 to 1500 C. The final sinteringtemperature and tune Wlll vary depending upon the most criticalcharacter stic desired in the final capacitor. Thus, for example, whenhigh reliability is of utmost importance, the temperature and time areextended (eg the temperature may be as high as 2350 C. and the finalsintering time may be as long as 45 minutes). When maximum capacitanceper gram is the objective, the final sintering temperature is preferablyabout l875 0-1950 C. and the sintering time is only about 30 mintues.

For a fuller understanding of the nature and objects ofthe invention,reference should be had to the following detailed discussion thereoftaken in connection with the accompanying drawing in which:

FIG. 1 is a cross-sectional view of the mold sintering support formaking the tantalum anodes;

FIG. 2 is a top view of the mold sintering support for making thetantalum anodes; and

FIG. 3 is a fragmentary enlarged sectional view of a recess in the moldsintering support for making a tantalum anode showing the anode as itappears after sintering.

In the production of capacitors having sintered tantalum anodes, it hasbeen ascertained that the capacitance per unit weight, with a givenbatch of tantalum powder, is inversely proportional to the density ofthe pellets from which the sintered anode is formed for a givencondition of sintering. The capacitance also decreases with increasingsintering temperature and time. It has also been ascertained that theESR decreases with decreasing density of the pellet. Heretofore, toobtain maximum capacitance per unit weight and minimum ESR, it wasnecessary to compress the powder to a pellet having a density which wasat th minimum consistent with sufiicient pellet strength to resist thehandling incident to the subsequent vacuum sintering operation. Thisminimum practicable density thus determined, and limited, the highestobtainable capacitance per unit weight and lowest obtainable ESR for agiven powder for anodes made by prior art techniques (for a given set ofsintering conditions).

Referring to the drawings, there is illustrated in FIGS. 1, 2 and 3 onepreferred apparatus and process for practicing the present invention. Ameasured quantity of tantalum metal powder 10 is shown in a verticalcylindrical recess 12 formed in a metal support 14 which serves as amold for sintering the metal powder. A tantalum lead wire which isinserted into the tantalum powder is illustrated at 13. The mold support14 preferably comprises an upper member 16 which is a cylindricalsection consisting of tantalum metal. In member 16, cylindrical recesses12 are preferably formed by drilling so as to serve as the moldchambers. A bottom member 18 is shown as a plate of tantalum metal whichis separable from member 16 and forms the base of the mold chambers 12.

.In the preferred form illustrated the upper chamber 16 is suitably cut,as at 17, to provide for ease of separation to permit ready removal ofthe sintered anodes. The various sections can be suitably held togetherby a clamp (not shown) for ease of handling as a complete mold assemblyas indicated at 14. While this is a preferred form, a solid upper member16 having the cylindrical recesses Would be equally suitable. A desiredamount of tantalum powder 10 is poured into each mold chamber 12 and alead wire 13 is inserted in the mass of powder.

This powder may be lightly tamped to assure good contact with the leadwire, but the tamping is not sufficient to compact the powdersubstantially from the maximum volume that can be assumed by the free,unsintered powder. The mold assembly 14, containing the tantalum powder,is then placed in a vacuum sintering furnace, only for a time and at atemperature sufiicient to form a coherent pellet. After this step thepellet is removed and then subjected to a further sintering operation.

When the first sintering operation is complete the mold assembly 14 iscooled and then removed from the furnace. Where the upper member 16 issectional, the sections are disassembled and the sintered anodes areremoved. When the upper member 16 is not sectional, but a single member,it is removed, separated from the base 18 and the sintered anodes arepushed out of the recesses with a plunger.

The invention will now be described by way of specific non-limitingexamples thereof.

EXAMPLE 1 The tantalum metal powder used had an average particle size onthe order of 25 to 30 microns, the preponderance of the powder having aparticle size between and 30 microns. The powder was also substantiallyfree of all volatilies which could be released in a vacuum at 500 C. Theindividual particles of this powder were generally solid and roughlyspherical in shape. They did not include many sintered agglomerates norstringy particles. A fraction having a particle size of about 5 tomicrons was separated, using a Roller particle size analyzer (Cat. No.5451, American Instrument Company). This 5 to 10 microns fraction had anuncompacted minimum bulk density of about 5.0 gm./cc. 1.8 grams of thistantalum powder fraction was loosely disposed in a inch diameter by /2inch long cylindrical mold chamber in a tantalum support. The bulkdensity of the powder so disposed was approximatley 5 .0 gm./ cc. Atantalum lead wire was next inserted in the tantalum powder. Thetantalum support containing the tantalum powder and lead wire was thenintroduced into the vacuum sintering furnace. The initial sintering wascarried out for a period of M1 hour at a pressure between about 5 10-and 12 10 microns Hg abs., and at a temperature of about 2150 C. Thecoherent pellet thus formed was then removed from the mold aftercooling. The next step was to place the pellet on a tray and introduceit into the final vacuum sintering furnace for further sintering for aperiod of /2 hour at a pressure of between about 5 10 and 12x10 micronsHg abs. and at a temperature of about 2150 C. These sintering conditionsprevent the density of the sintered mass from rising above 10 gm./cc.The next step was to form an oxide film having dielectric properties, onthe sintered tantalum electrode. This was done by anodic oxidation ofthe pellet in a 0.01% phosphoric acid solution at 90i2 C. at anapproximate constant current density of 35 milliamps per gram of pelletweight until the desired voltage of 200 was reached after which theformation, at constant voltage, was continued for 2 hours, followingwhich it was rinsed for 30 minutes in distilled water and dried. Theelectrical characteristics of the anodized pellet are reported in thetable. The standard conditions of measurements of DC. leakage,capacitance, and ESR (referred to in the specification and claims asstandard ESR conditions etc.), were done using 10% phosphoric acid atroom temperature as electrolyte and a cylindrical platinized silvercathode 1.5 inches in diameter and 2 inches in height. D.C. leakage wasmeasured after 2 minutes at 140 volts and capacitance and 'ES'R weremeasured at 120 cycles per second with a 0.5 volt A.C. signal. Animpedance bridge (for example, Type 1650-A, General Radio Company) wasused to measure the dissipation factor (B) and capacitance (C); and theESR was calculated using the relationship where C is the capacitance (infarads), F is the frequency of the AC. signal (in cycles/second), andESR is given in ohms. Breakdown voltage was measured by continuing theforming step (i.e. the anodic oxidation of the pellet in 0.01% H PO ati2 C. at constant current density of 35 ma./gm.) until breakdown of thedielectric film occurred.

EXAMPLE 2 In this example, the tantalum powder used was similar to thatof Example 1 but had an average particle size of 4.5 microns (Fishernumber) with a preponderance of the particle size between 5 and 15microns '(Roller analyser). This powder had an uncompacted minimum bulkdensity of about 3.0 gm./cc. 1.8 grams of this powder was looselydisposed in the same mold as used in Example 1. The bulk density of thepowder so disposed was approximately 4 gm./cc. A tantalum lead wire wasnext inserted in the tantalum powder. The tantalum support containingthe tantalum powder was then introduced into a vacuum sintering furnace.The initial sintering was carried out for a period of 20 minutes at apressure between about 5 10 and 12x 10 microns Hg abs. and at atemperature of about 1400 to 1500 C. The coherent pellet thus formed wasthen removed from the mold after cooling. The pellet was then placed ona tray and introduced into the vacuum sintering furnace for furthersintering for a period of /2 hour at a pressure of between about 1 10and 10x 10* microns Hg abs. and at a temperature of 2000 C. The sinteredpellet was then anodized and tested electrically by the same standardconditions as outlined in Example 1. The results of the electrical testare reported in Table II.

ESR=

EXAMPLE 3 In this example the tantalum powder used had an averageparticle diameter of 10 microns and a particle size distribution ofbetween 62 microns and 5 microns. This powder had been obtained byhydriding and grinding tantalum chips. 1.8 grams of the tantalum powderwere loosely disposed in the same mold as used in Examples 1 and 2. Thetantalum powder so disposed was sintered at a temperature of about 1500C. for 20 minutes as in Example 2. It was then removed, as a coherentbody, from the mold and sintered at about 2150 C. for /2 hour at apressure of between about 1X10- and 1x l0 microns Hg abs. The resultantpellets were formed to 270 volts instead of 200 volts as described inExample 1. The 270 volt formation test consists of forming anodes at acurrent density of 35 ma./ gm. in 0.01% H 'PO at 92 C. to 200 volts. Thevoltage is held at 200 volts until the current decays to of the originalvalue. Formation is then continued at constant current to 270 volts andterminated after a one-hour hold period. The DC. leakage is measured in0.01% H PO at 25 C. at a test voltage of 240 volts. Capacitance ismeasured in 10% H PO Additional anode pellets were produced inaccordance with the procedures outlined above in Examples 2 and 3. Anumber of important parameters concerning those anodes, along withExamples 1, 2 and 3, are given in Table I below. In this connection, theaverage particle diameter is expressed as Fisher Average ParticleDiameter (FAPD) as more fully described in Ind. Eng. Chem. Anal, Ed. 12,479-482 (1940), E. L. Gooden, and C. M. Smith. The Powder Type refers tosource of the tantalum powder; D means powder resulting from thereduction of potassium fluotantalate by sodium; H means powder resultingfrom the hydriding, grinding and dehydriding of tantalum chips.

TABLE I Fisher Final Average Final Sinter Particle Powder Sinter Time,Sinter Formation Diameter (41) Type Temp.,C. minutes Density Voltage4.5 1) 2,150 30 8.6 200 4.5 D 2, 000 30 8.34 200 9.5 H 2,150 30 11.3 2709.5 D 2,350 45 7.5 270 0.5 H 2,150 30 10.0 200 0.5 H 2,150 30 11.0 2004.5 H 2,000 30 10.1 200 0.5 H 2,025 30 9.1 200 9.5 D 2,150 30 0.4 2009.5 D 2,150 30 9.5 200 9.5 1) 2,200 45 7.2 270 0.0 1) 2,200 45 9.7 2704.5 D 2,000 30 8.2 200 4.5 D 1,950 30 8.0 200 4.5 D 1,000 50 9.0 200 4.5H 1,875 30 9.3 200 The sintered and formed anodes described in Tablebetween Examples 5 and 5A shows the great improvement I above weretested as described in Examples 1 and 3 in anode obtained applying thepresent invention to a above. The results of such tests are given inTable II hydrided and ground product. As noted, the electrical b low,properties of the anode of Example 5 are greatly superior to those ofExample 5A, which is formed by conventional techniques. Similary,Example 12 should be compared TABLE II with Example 12A wherein an anodeformed in the pres- C 01 CV Dr ESR L/C BDV ent invention 12 is comparedwith an anode made by 18.0 155 3 600 122 (m 0.07 conventional techniquesin 12A. The anode of the present 20.7 172 4,140 17.3 0.2 0.110 234-270invention has greatly superior electncal properties, as can 7.0 80 2,05012.4 8.0 .27 300 be seen 10.8 81 2,100 0.4 5.8 .25 255 12.5 132 2,50021.3 9.2 .00 300 While several preferred embodiments have been del a: 1%3 3g? 13:; :8; 23 g scribed above, they should be taken as illustrativeof the 15.1 137 3,020 20.0 7.1 .08 200 invention rather than as beinglimiting. In this connection, 12 5188 g $13 :3; 3% it is certainly notnecessary to use a tantalum mold. The 0-0.5 0545s 2, 430-21 500 5.2 4.:.3-. mold can be formed merely with a tantalum foil lining, 3 2 ig? ,3 22 8: 23mm or it can even embody the use of graph1te, alundum, zir- 1004,900 17.5 5.2 .00 240 coma and other refractory materials. It 18particularly is; 2 833 3 2 :2 2%? important when in using materialsother than tantalum k in contact with the powder, to limit the initialsintering gjgi gggfgflgggggi temperatures to between 1400 and 1500 C. toprevent Dt=Dissipation factor, percent. contamination of the tantalumpowder. gff giggf gg?gggig fi ggf Volts/gr Since certain changes may bemade in the above product ESR=Equivale11t series resistance, ohm. andprocess without departing from the scope of the in- BDv=BmakdwnvmgbWitsvention herein involved, it is intended that all matter contained in theabove description shall be interpreted as illustrative and not in alimiting sense. As can be seen from a study of Tables I and II, thepreswh is Claimed i ent invention provides for the production ofcapacitors 1 The process f f i an anode f an electrical havlhgpropbrttes Phobtalhable by other qh when capacitor which comprises thesteps of introducing tantaextremes of rellabihty are the ob ect1ve,along with reasonl powder into a mold to provide a loosely disposed yhlgh capacltance, the hlghel slhtel'lhg pb a res mass of said powderhaving no cohesive strength and a of Exfimples 4 and 5 are p When theultlmhte 1h bulk density of about 3 to 6 grams/cc., said powder havlgh tP vbhlme and pf h P g 15 the ing an average particle diameter of lessthan 10 microns, oblectlve, the l'elatlvely lbwef slhtetlhg temperaturesof heating said powder to form a coherent self-supporting Examples l1,l2 and 13 are preferred. pellet, removing said pellet from said mold andfurther Paftlclflal" attehtloh Should be glvefl to certain adVaIlvacuumsintering said pellet at a temperature of about tag$ Yvhlch can bobtalhed y the Process of the Present 1875 C. and above to form ahigh-capacitance, low-leakinvention as applied to dlfierent types oftantalum powder. age intered d v In thls Connection, Example 3 hh y aheXlJeQSb/e P 2. The process of forming an anode for an electrical def($t P p y hydrlfhhg and gr111d111g t capacitor which comprises the stepsof introducing tantalum chips. Thls gives a high reliability, hlghcapacitance lum powder into a mold to provide a loosely disposed anodewhen lnt r at 215O In EXample mass of said powder having no cohesivestrength and a Prlced dlfectly leducfid P f b slhteled bulk density ofabout 3 to 6 grams/cc., said powder havt 2350 C b 45 mlhutes elves ahanode havlng h h y ing an average particle diameter of 4 to 6 microns,heating higher capacitance per un t volume, lower dissipation saidpowder to form a coherent self-supporting pellet, refactor, lower ESRand equivalent L{C. The only dlsadmoving said pellet from said mold andfurther vacuum vantage 1n the use of thls less expensive powder is 1nthe sintering said pellet at a temperature of about 1875 C.- breakdownvoltage of Example 4 WhlCh 1s 255 as comand above to form ahigh-capacitance, low-leakage sinpared with above 300 volts for Example3. This same tered anode. powder is used in Example 8, sintered at 2150and is 3. The process of forming an anode for an electrical comparedwith the same powder formed into an anode capacitor which comprises thesteps of introducing tantaby' convent1onal techniques as described 1n8A. In this lum powder into a mold, heating said powder to form aconnection, the anode of Example 8 has a CV which is coherentself-supporting pellet, removing said pellet from substantlally greaterthan the CV of the conventional said mold and further vacuum sinteringsaid pellet at a anode (8A) the Df is substantially less, as is the ESR,temperature of about 1875 C. and above to form a high- Whlle the otherproperties are equivalent. A comparison capacitance, low-leakagesintered anode, said sintering step being conducted at a temperature ofabout 1875 1950 C. for about 30 minutes to form an anode having acapacitance in excess of 4000 y. fv./ gr.

References Cited UNITED STATES PATENTS 2,198,702 4/1940 Koehring 15-222,299,228 10/1942 Gray et a1. 317230 8 3,004,332 10/ 1961 Werner 317-2303,093,883 6/1963 Haring et a1. 317--230 FOREIGN PATENTS 5111805 8/1939Great Britain.

JAMES D. KALLAM, Primary Examiner US. Cl. X.R. 317-23O

